Static mixers or motionless mixing apparatus are widely employed to provide effective mixing and/or flow conditioning of one or more fluids flowing within a fluid containment and transport vessel, such as a circular pipe, and the like. The general technique employed by the previously known and used mixers and mixing apparatus was to divide the flow into a series of smaller, separate flow streams within the pipe or other vessel. These flow streams are then forcibly diverted away from neighboring flow streams and into proximity of more distantly removed flow streams Division into the series of separate flow streams is accomplished through the use of extensive series of baffles or spiraled inserts of rigid material inserted into the flow path. By inserting the baffles or spiraled inserts into the flow path, the flow streams are divided, and then divided again, until the entire flow is a plethora of intertwined flow streams. The intertwined separate flow streams will intermix due to the viscous characteristics and effects of the fluid.
The degree of effectiveness of these earlier static mixers and mixing apparatus varies with the particular design. The penalty one commonly pays with static mixers, however, is a significant increase in flow pressure and energy losses due to the excessive interference of the mixer or mixing apparatus with the main fluid flow. Additionally, since motionless mixers generally employ a large number of baffles or other convoluted pieces of rigid material placed in the path of the flow stream, this creates a number of surfaces upon which material suspended in the fluid may collect, causing the mixer to foul and plug during operation. The motionless mixers are generally effective in promoting fluid mixing, but do so at the significant expense of increased pressure and energy loss, and the increased necessity for frequent cleaning and/or replacement of the mixer.
Unlike prior static mixing approaches which are basically brute force techniques, the present invention relies on the implementation of more natural mixing processes which revolve about the controlled generation of vortices, or swirling motions in the flow. The natural character of a turbulent flow is to generate streamwise (flow direction) vortices in a somewhat organized fashion such that the swirling motions cause the movement of fluid perpendicular to the main flow direction. This is the physical process responsible for fluid flow mixing. When present in sufficient number and dispersed throughout the fluid flow, the effect of the streamwise vortices is to produce adequate cross-stream mixing (i.e. mixing due to the intermingling of fluid in directions perpendicular to the main flow direction).
It has been determined that the key to producing crossstream mixing, however, is that the vortices must be oriented in the main flow or streamwise direction to be effective. When the vortices are of such a streamwise orientation, they tend to push fluid away from the sides of a bounding surface (e.g. The wall of a pipe) and into the flow away from the surface (the outer flow). In such an orientation the vortices also pull fluid from the outer flow toward the bounding surface (e.g., the pipe walls). This alternating push-pull effect results in the cross-stream motion of alternating regions of inflow and outflow in proximity to a bounding surface creating a rich intermingling of the flowing fluid, and hence, mixing.
Clearly, the key point in the development of the static flow mixing apparatus of the present invention is the generation or artificial creation of flanking vortices oriented in the direction of the main flow with each vortex swirling in a direction opposing the direction of swirl of the adjacent vortices. The resulting flow pattern from these vortices is the creation of alternating "channels" across the flowing fluid within which the flow moves in opposing cross-stream directions.
In nature, these streamwise vortices do not remain in a stable configuration, due to the strong interaction between the adjacent, counter-rotating streamwise vortices. This interaction rapidly destabilizes the streamwise vortices such that a spanwise "connection" develops between the counter-rotating pair of streamwise vortices. These connections between the streamwise vortices rapidly create what appear as a continuous progression of arch-shaped vortices, commonly referred to as horseshoe or hairpin vortices. The development of these connecting arches does not alter the basic mixing effectiveness of the streamwise vortices, but appears to augment it by creating "subdivisions" in the movement of fluid pumped away from the surface, which facilitates an even more effective intermingling of the fluid regions being pushed away from the surface and pulled toward the surface by the action of the alternating streamwise portions of these vortices. As these arched vortices evolve, they undergo a complicated process of three-dimensional intertwining and agglomeration which facilitates the rapid dissipation of the original vortex pattern; the combination of the induced cross-stream mixing and the three-dimensional dissipation of the original vortex pattern promotes the rapid development of homogeneity of momentum and species concentration within the fluid.
Nature, left to its own devices, does an adequate job of creating similar conditions in a turbulent flow. The static flow mixer of the present invention assists this naturally occurring mixing by creating streamwise vortices in sufficient strength, spacing, and orientation such that the flow mixing process is substantially amplified and greatly accelerated.
Once the alternating streamwise vortex flow pattern is produced by the mixer, the natural interaction of the vortices with both the surrounding fluid and each other produces the desired mixing. Continued division and recombination of the flow (as is required in prior static mixing approaches) is not required.
The static mixing created by the present invention promotes the efficient circulation of fluid both towards and away from a bounding surface, which enhances not only fluid mixing, but also increases momentum and energy transport within the fluid as well as increasing the transfer of heat to or from the bounding surface by the flowing fluid. To understand the resulting effects of the fluid mixing one must recognize that a moving fluid has certain properties which are carried with it. Examples of these properties are the mass of the fluid, momentum (i.e. proportional to the velocity of the fluid), kinetic energy (proportional to the square of the velocity of the fluid), internal thermal energy (characterized by the temperature of the fluid), and species (any material mixed with the fluid. e.g. dissolved salts or dyes in water, water vapor or smoke in an airflow). Thus, when the fluid is caused to move perpendicularly to the main flow direction, the resulting cross-stream movement carries all of the above properties with the fluid. The interaction of this cross-stream flow with the surrounding fluid causes an exchange and intermingling of the fluid properties throughout the fluid. Thus, not only does the cross-stream motion set up by the mixer of the present invention cause the fluid to mix, but it also causes a mixing of the velocities (momentum), the kinetic energies, the fluid temperatures (i.e. thermal energies), and the transported species. The cross-stream mixing causes the resulting mixed fluid to take on the "average" of the properties of the mixed fluid streams. For example, if a high temperature fluid enters and mixes with a region of lower temperature fluid, the resultant temperature of the mixed fluid will fall somewhere between the high and low temperatures. If the process of cross-stream fluid movement is greatly accelerated, such as is accomplished with the mixing apparatus of the present invention, the fluid properties in the main flow will become more homogeneous and uniform more quickly. The complete mixing of a fluid flow would result in the complete identity of any point within the flow with any other for each of the flow properties described above, i.e. mass flow, velocity, temperature, and species.
In addition to the mixing of fluid properties across a flow, the cross-stream movement of fluid in proximity to a solid boundary, e.g. a pipe wall, will result in the increased transfer of heat from the boundary material to the fluid, or from the fluid to the boundary material. The amount of heat which will be transferred to or from a surface, such as the cooled or heated wall of a pipe, depends directly upon the difference in temperature between the wall of the pipe and the fluid directly adjacent thereto.
Normally, for a laminar flow, the fluid near the boundary surface is very close to the temperature of the boundary material, resulting in low heat transfer. If the flow is more turbulent, there is a cross-stream flow pattern set up which brings fluid from the center of the vessel or pipe toward the boundary surface and carries fluid away from the boundary surface toward the center of the vessel. This interaction results in a greater temperature difference, on the average, between the boundary surface and the fluid adjacent to that surface. Thus, a greater thermal energy exchange will occur. The same process applies for the transfer of species to and from the boundary surface and the center of the vessel, and vice versa.
Thus, the static mixing apparatus of the present invention can be used effectively to mix a flowing fluid to yield substantially uniform velocity, energy, and species concentration and to significantly increase the amount of thermal energy transferred between the fluid and the boundary surface material. This increase in uniformity of the various properties of the fluid demonstrates the equalization of the distribution of each of these properties throughout the fluid by the static mixing apparatus.
It is therefore an object of the present invention to provide a static mixing apparatus, of the motionless type, which will substantially increase the various properties of the fluid to a uniform distribution and/or concentration.
It is a further object of the present invention to provide a static mixing apparatus which is self-cleaning and non-plugging present invention
It is still a further object of the present invention to provide an accelerated cross-stream mixing of the fluid in a significantly reduced streamwise distance.
It is another object of the present invention to provide a greater thermal exchange between the fluid and the boundary material by creating alternating flows of fluid both toward and away from the boundary surface and the center of the containment and transport vessel due to the action of vortices swirling in a streamwise direction.
It is still another object of the present invention to provide greater cross-stream mixing of a fluid in the streamwise direction to achieve more uniform flow properties in fluid.
Other objects will appear hereinafter.